Preparation of oxygen electrodes for fuel cells

ABSTRACT

Two layers where each contains a skeleton-forming fluorinecontaining resin and only the first layer contains silver carbonate are pressed together to a disc. At least the first layer contains a water-soluble salt which is leached out to form pores. The silver carbonate is reduced to catalytically active metallic silver. In the obtained electrode, the first silvercontaining layer is electrically conductive and wettable by the electrolyte, the second layer is hydrophobic and its pores suitable to retain the oxygen.

United States Patent [72] Inventors Germany [21] Appl. No. 659,619 [22]Filed Aug. 10, 1967 [45] Patented Dec. 28, 1971 [73] Assignee RobertBomh Gmbll Stuttgart, Germany [32] Priority Nov. 2, 1966 3 3 Germany [31B 89654 [54] PREPARATION OF OXYGEN ELECTRODES FOR FUEL CELLS 7 Claims,No Drawings [52] U.S. Cl 136/120 FC, 75/201, 75/208 [51] Int. Cl H0lm13/04, 822!" 7/00 [50] Field of Search 136/ 1 20;

75/208, 201; 264/22, 49, ll2,l13,l27

Primary Examiner-Winston A. Douglas Assistant Examiner-M. .l. AndrewsAttorneys- Fritz G. Hochwald and Christen & Sabol ABSTRACT: Two layerswhere each contains a skeletonforming fluorine-containing resin and onlythe first layer contains silver carbonate are pressed together to adisc. At least the first layer contains a water-soluble salt which isleached out to form pores. The silver carbonate is reduced tocatalytically active metallic silver. in the obtained electrode, thefirst silver-containing layer is electrically conductive and wettable bythe electrolyte, the second layer is hydrophobic and its pores suitableto retain the oxygen.

PREPARATION OF OXYGEN ELECTRODES FOR FUEL CELLS This invention relatesto the preparation of silver-containing cathodes for fuel cells.

For operating fuel cell cathodes (oxygen electrodes) with alkalineelectrolytes, silver has been a preferred catalyst. The silver can beemployed as powder or in form of a powdered Raney alloy with aluminumwhich is later dissolved out, or as a compound which is decomposed atsintering temperature (German DAS No. 1,174,861) and formed to asintered body (German Pat. No. 1,197,941). Thereby, the silverconsumption is rather high. The silver content can be reduced by mixingthe silver with another metal, e.g., nickel, in form of powtiers and tosinter the mixture to a porous body (US. Pat. No. 3,020,327). Suchelectrodes have a high weight, and on sintering undesirable alloyformation may take place which reduces the activity.

It in also known to incorporate silver or a Raney silver alloy us powderin a resin matrix; this can be done by mixing the silver or Raney alloywith a powdered or granular resin and pressing the mass (U.S. Pat. No.3,134,697; French Pat. No. 1,397,092; German DAS No. 1,219,105) or byforcing suspended catalyst particles into a porous matrix (U.S. Pat. No.3,171,737; British Pat. No. 986,324). Thereby, the use of Raney silverhas particular advantages due to the large inner surface of the spongymaterial. However, the preparation of the catalyst is complicated, thedisintegration of the ductile alloys is often difficult, and thecatalyst, before or after it has been formed, must be activated byleaching out the aluminum.

According to various procedures, porous bodies of carbon or electricallynonconductive materials can be impregnated with solutions of silversalts whereupon the silver is chemically reduced or electrolyticallyprecipitated (German DAS No. 1,171,482; British Pat. Nos. 995,903 and986,324). Said impregnating methods present the advantage that thesilver is deposited in finely divided form in the matrix but they havethe drawback to render the entire matrix, even though it consists ofdifficulty wettable material, wettable and hydrophilic so that it can beutilized only under pressure as a gas electrode.

The reason is that in fuel cells for free (liquid) electrolyte andgaseous fuels, the electrolyte must not enter the gas space through thepores of the electrode. This can be accomplished by applying asufficiently high gas pressure, or more simply by rendering the layer ofthe electrode which faces the gas space, water (i.e. electrolyte)repellent. However, the catalyst-containing layer must remain accessiblefor the electrolyte to permit the ions formed in the electrochemicalreaction to pass into the electrolyte. Methods to impregnate porousbodies with suitable materials (paraffin, hydrophobic polymers) areknown; however, when only part of the body is to be impregnated, theyrequire a careful and complicated procedure.

A primary object of the invention is to provide a simple method forpreparing a porous electrode one part of which is electricallyconductive, contains a catalyst and is wettable by the electrolyte whileanother contiguous part is nonwettable and filled with gas.

Other objects and advantages will be apparent from a consideration ofthe specification and claims.

According to the invention, these objects are accomplished by joining anelectrical conductivity ensuring mixture of powders of silver carbonate,a fluorine-containing resin, and one or more water soluble salts underheat and pressure with a layer which consists only of afluorine-containing resin or of a mixture of such resin with a watersoluble salt, to a disc and making the disc catalytically active byreducing the silver carbonate with hydrogen and porous by leaching outthe salts. With respect to metallic silver, which could also be used,the silver carbonate has the advantage that, on reduction, an additionalsystem of micropores is formed, which renders the catalyst particularlyactive. In addition, the carbonate reacts with the resin at least at thecontact points of the powder particles. For the proper operation of theelectrodes prepared in accordance with the invention, it is ofimportance that the catalyst-containing layer, though consisting largelyof hydrophobic material, is, by the formed silver and additionally bythe interaction with the carbonate at sintering temperature, madewettable by the electrolyte to such an extent that the catalyst can takepart in the electrochemical reaction, while the back of the electrodewhich consists only of resin, remains completely unwettable and filledin all pores with oxygen or air.

Thereby, it is surprising that the fluorine containing decompositionproducts of the resin, which may be formed in the sintering operation,do not convert the silver to silver fluoride and deactivate it, but thatsaid finely distributed metal imparts to the mass a high electricconductivity.

If, due to the use of an insufficient amount of silver carbonate, theconductivity of the mass remains too small, an alkali-resistantconductive component, e.g., nickel powder, can be added to the mixtureof the starting materials to bring the conductivity to the desiredlevel.

The preparation of the electrodes will be described in detail withreference to the examples 1 to 4.

The active layer of the electrode consists of a mixture of powders of afluorine containing, i.e. hydrophobic, resin, silver carbonate, and awater soluble pore-forming material. if desired, an alkali-resistantconductive substance may be added.

By grinding in a mortar, the powders are mixed for a sufficient periodof time that a ductile scaly mass is obtained which is then evenlydistributed in a die cavity. On this layer, the mixture of the backlayer is spread, which mixture contains pore-forming agents and resinpowder (examples 1 and 2). The mixtures are compressed under a pressureof 1-3 Mp./crn. to a shaped body.

If the backlayer is to be prepared without a pore forming agent(examples 3 and 4), we prepare first under a pressure of l to 3 Mp./cm.a pressed body only of the front layer, whose one face is roughened. Theresin powder evenly spread on said roughened surface is then compressedunder a pressure of l to 5 kp./cm. only so far that about 50 percent byvolume of pores are still contained in the mass.

Thereby, a preformed wire of an alkali-resistant material, e.g., silver,can be also pressed into the front layer, to produce a better contact ofthe electrode.

The satisfactorily coherent pressed body is placed between porousceramic discs and loaded with a weight of about 300 g. During 1 hour,the silver carbonate is reduced at a temperature of 200 C. in a hydrogenatmosphere to metallic silver; subsequently, the temperature isincreased to 370 C. so as to sinter the resin particles together. At thesame time, the silver catalyst particles sinter together and form aconductive.

skeleton in the electrode body.

If, as described in example 4, polytrifluorochloroethylene is used asthe skeleton material, a temperature of 270 C. is sufficient for thesintering of the resin but not for the bonding of the silver particles.For this reason, a very fine nickel powder is added so as to producemechanically as many bridges between the individual catalyst particlesas possible.

Finally, the pore forming agent is leached out of the activated andsintered electrode discs with water, the temperature of which may beincreased to boiling, and the electrode is dried.

As shown in example 4, sodium carbonate can be incorporated into theactive layer as pore forming agent. This is of advantage because thesodium carbonate reacts on sintering with the fluorine containing resin.The resin is so modified as to lose its hydrophobic character and toallow the electrolyte to penetrate into the active front layer. Theporous hydrophobic back layer of the electrode, which serves also assupporting layer, ensures access of the oxidizing agent (oxygen, air) tothe catalyst for the electric reaction.

The thus prepared electrodes can be operated in test cells or batteriesin the presence of an alkaline electrolyte at temperatures up to 200 C.

Particularly for operation with oxygen contained in air, the supportinglayer must be of such thickness. and its pores must be of such size thatsaid supporting layer will have the required strength and be impermeableto the electrolyte. because a pore system, which is too narrow. wouldretain a nitrogen cushion which would interfere with the oxygen access.

EXAMPLE l into a die cavity of 24 mm. diameter, there are sequentiallyplaced the following mixtures:

l. 2.0 g. ofa mixture consisting by volume of 45 percent of silvercarbonate, grain size 5 um.

25 percent of polytetrafluoroethylene, grain size 45 30 percent ofsodium chloride, grain size 30-60 gm.

2. 1.5 g. ofa mixture consisting by volume of 30 percent ofpolytetrafluoroethylene. grain size 45 70 percent of sodium chloride,grain size 30-60 m.

The mass is compressed under a pressure of 3 Mp./cm.'-. The pressed bodyis placed between ceramic discs and loaded with a weight of 300 g. Thesilver carbonate is reduced for 1 hour in a hydrogen atmosphere at 200C. to silver. Subsequently. the body is sintered for 2 hours at 370 C.

The electrode contains 240 mg./cm. of silver. After leaching out thepore-forming agent in water, the temperature of which may be increasedto boiling. the electrode is tested in a half-cell arrangement withoxygen (air) as fuel without applying pressure. When 65 n KOH at 70 C.is used as electrolyte, the electrode with reference to a hydrogenelectrode in the same electrolyte produces the following voltages:

Open-circuit 50 maJcrn. I Ind/cm. voltage Oxygen L080 mv. 920 mv. 830mv. Air L060 mv. 800 mv. 640 mv.

EXAMPLE 2 Into a die cavity, as described in example I, the followingthree mixtures are filled one after the other:

i. 0.7 g. of a mixture consisting by volume of 80 percent silvercarbonate, grain size um. percent polytetrafluoroethylene. grain size 45pm. 2. 1.5 g. ofa mixture consisting by volume of 40 percent silvercarbonate, grain size 5 um. percent polytetrafluoroethylene. grain size45 um. 35 percent sodium sulfate, grain size -60 pm.

3. 1.5 g. ofa mixture consisting by volume of 30 percentpolytetrafluoroethylene. grain size 45 um. 70 percent sodium sulfate,grain size 30-60 pm.

The mass is treated as described in example I. On reduction of thesilver carbonate, the front layer l develops very fine pores which, dueto the capillary forces, fill well with electrolyte. The middle layer(2), which contains additionally large pores. is a quasi-transition tothe hydrophobic back layer (3) which does not contain a catalyst.

The layers (1) and (2) have a combined content of 280 mg./cm. of silver.In a half-cell arrangement as in example I, the following voltages foroxygen and air at 3-5 cm. water column above atmospheric pressure in 6.5n KOH and 70 C. are measured:

Open-circuit S0 malcm.

voltage Oxygen L070 mv. 910 mv Air L040 mv 790 mv.

EXAMPLE 3 In a die cavity of 48 mm. diameter. there are placedsequentially:

l. About 2 g. ofsodium chloride. grain size 30 am. 2. 8.0 g. ofa mixtureconsisting by volume of 45 percent silver carbonate. grain size 5 um. 25percent polytetrafluoroethylene. grain size 45 pm.

30 percent sodium chloride. grain size 30-60 pm.

The mass is pressed under a pressure of l MpJcm. and the ram is removedfrom the die. A backlayer (3) which consists of (3) 2.0 g. ofpolytetrafluoroethylene. grain size 45 um. and does not contain a poreformer, is loosely spread on the prepressed and roughenedsilver-containing layer (2), and then pressed thereon under a pressureof5 kp./cm.. This pressure is low enough to compact thepolytetrafluoroethylene layer to about 50 percent.

As described in example I. the obtained matrix is activated, sintered.and the sodium chloride of the base layer as well as the pore former areleached out. In a half-cell assembly as in example I. the followingpotentials in 6.5 n KOH at C. are measured when the electrode, whichcontain 240 mgJcm? of silver, is operated without application ofpressure:

Open-circuit 50 maJcm. potential Oxygen L080 mv. 920 mv. Air L060 mv 8l0mv.

EXAMPLE 4 into a heatable die of 48 mm. diameter, there are filledsequentially:

1. I00 g. ofa mixture containing by volume 20 percent silver carbonate,grain size 5 um.

25 percent polytrifluorochloroethylene powder, grain size 20 um.

25 percent nickel powder. grain size l-5 um.

15 percent sodium chloride, grain size 30-60 pm.

15 percent sodium carbonate, grain size 30-60 am. 2. 2.0 g. ofpolytrifluorochloroethylene powder, grain size V.

The catalyst containing layer (l) is first pressed under a pressure of lMp./cm. After removal of the ram. the powder (2) for the backlayer isspread on layer (1) and pressed thereon under a pressure of 5 kp./cm.The pressed body is sintered for 1 hour at 270 C. while said pressure of5 ltp./cm. is maintained.

The silver carbonate is then at l50 C. during 5 hours reduced in ahydrogen atmosphere to active silver. The pore formers sodium chlorideand sodium carbonate are leached out with warm water. The electrodecontains 100 mg./cm. of silver.

The sodium carbonate is added as pore former to the sodium chloridebecause it reacts in direct vicinity with thepolytrifluorochloroethylene; hereby the resin skeleton loses itshydrophobic character. and the active layer is better wetted by theelectrolyte.

In a half-cell arrangement as in example 1, the electrode when operatedwithout application of pressure with oxygen (air) in 6.5 n KOH at atemperature of the electrolyte of C.. produces the following potentials:

Open-circuit 50 malcm. potential Oxygen L070 mv. 860 mv. Air L050 rnv.730 mv.

ln the examples, the abbreviation p" designates pond,"

the latter term being introduced in the European physical literature todefine the unit of force and to replace insofar the formerly used gram."Therefore, kp.=kilogram and Mp.- metric ton.

We claim:

l. A method for preparing a porous oxygen electrode for fuel cellsoperated with an alkaline electrolyte comprising forming at least oneactive substantially hydrophobic layer of a potentially electricallyconductive mixture of powders of silver carbonate, a fluorine containingresin, and at least one water-soluble salt other than silver carbonate,forming on said active layer a substantially hydrophobic supportinglayer of a member of the group consisting of a fluorine-containing resinand a mixture of said resin with at least one water-soluble salt otherthan a carbonate, compressing said layers to form a disc.

reducing the silver carbonate in said disc to catalytically activemetallic silver and sintering said disc to render said active layerelectrically conductive and electrolyte wettable, said supporting layerremaining substantially hydrophobic, and leaching out the water-solublesalts to render the disc porous.

2. The method as claimed in claim 1 wherein said fluorinecontainingresin is polytetrafluoroethylene.

3. The method as claimed in claim 1 wherein said fluorinecontainingresin is polytrifluorochloroethylene.

4. The method as claimed in claim 1 wherein said powder mixture containsto 50 percent by volume of silver carbonate and to 50 percent by volumeof said resin.

5. The method as claimed in claim 1 wherein two superposed active layersare formed and the contents of silver carbonate and of said resin insaid two active layers are different.

6. The method as claimed in claim 1 comprising adding to said powdermixture an electrically conductive substance.

7. The method as claimed in claim 1 comprising initially compressingsaid active layer to form a pressed body, applying said supporting layerand compressing said layer and said body at a pressure less than thatused in forming said pressed body to form said disc.

2. The method as claimed in claim 1 wherein said fluorine-containingresin is polytetrafluoroethylene.
 3. The method as claimed in claim 1wherein said fluorine-containing resin is polytrifluorochloroethylene.4. The method as claimed in claim 1 wherein said powder mixture contains10 to 50 percent by volume of silver carbonate and 15 to 50 percent byvolume of said resin.
 5. The method as claimed in claim 1 wherein twosuperposed active layers are formed and the contents of silver carbonateand of said resin in said two active layers are different.
 6. The methodas claimed in claim 1 comprising adding to said powder mixture anelectrically conductive substance.
 7. The method as claimed in claim 1comprising initially compressing said active layer to form a pressedbody, applying said supporting layer and compressing said layer and saidbody at a pressure less than that used in forming said pressed body toform said disc.